Exhaust after-treatment systems are employed for reducing regulated emissions from an exhaust stream of an engine system. An exhaust after-treatment system can comprise one or more of the same or different types of after-treatment devices including, for example, diesel oxidation catalysts (DOCS), diesel particulate filters (DPFs), and lean NOx traps (LNTs), lean NOx catalyst, or other catalysts and/or adsorbents.
Diesel particulate filters, also known as particulate filters, particulate traps, soot filters or soot traps, can be employed to reduce the levels of particulates in an engine exhaust stream prior to its release into the atmosphere. The filter can optionally contain a catalyst material. Particulates in the engine exhaust stream are trapped and collected by the filter. Eventually the accumulation of particulates adversely obstructs the flow of the engine exhaust stream through the filter, causing the pressure drop across the filter to be undesirably high. Various in situ regeneration techniques have been employed to regenerate DPFs by burning off (oxidizing) and removing the particulate matter, thereby restoring the pressure drop across the filter to desirable levels. DPF regeneration can be done passively or by using specific active regeneration techniques. Almost all active filter regeneration techniques operate by raising the temperature of particulates collected in the filter to a temperature at which the particulates will oxidize rapidly in the presence of oxygen present in the engine exhaust stream.
In some cases the filter temperature can be increased to a value suitable for regeneration by using electrical or microwave heating or by using a hot flue gas stream produced by a burner. Other prior approaches to actively regenerate a DPF in situ involve adjusting the operation of the engine to increase the temperature of the engine exhaust stream. Examples of such techniques include throttling of the engine, and/or post-injection of fuel into the engine exhaust stream, for example, periodically introducing diesel or a hydrogen-containing gas stream upstream of the DPF. As the mixed gas stream travels through the DPF, the DPF is heated by combustion of the mixture which can be promoted by an optional catalyst located upstream of and/or within the DPF. The regeneration process is an exothermic process which can be initiated above a threshold temperature (for example, above about 550° C. for a DPF without catalyst and above about 400° C. for a DPF with catalyst), and requires the presence of oxygen in the engine exhaust stream. The regeneration process can be self-sustaining provided there are sufficient amounts of heat, oxygen and particulates. DPFs can also employ a segmented regeneration strategy in which a segment or portion of the DPF is regenerated while other segments are not being regenerated. Regenerating only a portion of the DPF at a given time can reduce the mass flow rate of fuel needed for regeneration, enabling a reduction of the size and cost of some system components.
Lean NOx traps (LNTs) can be employed to reduce the level of nitrogen oxides (NOx) in an engine exhaust stream prior to its release into the atmosphere. LNTs operate by employing adsorbents to adsorb NOx from the engine exhaust stream during lean (excess oxygen) conditions, and using a regeneration process in which NOx is desorbed from the absorbents and then converted during reducing or rich (excess fuel) and elevated temperature conditions. The regeneration process can restore the capacity of a LNT to adsorb NOx and typically is performed prior to reaching the adsorption capacity of the LNT. Creating a reducing environment, by removing most of the oxygen as well as introducing a reducing agent into the LNT, reduces the temperature at which regeneration will occur. Combusting a reducing agent can consume most of the oxygen and increase the temperature sufficiently for regeneration. Suitable reducing agents include, for example, syngas, hydrogen, diesel, carbon monoxide, or other hydrocarbon fuels.
Sulfur (S) species, originating from the engine fuel and oil, can be present in the engine exhaust stream. As the engine exhaust flows through a LNT, sulfur tends to be preferentially adsorbed over NOx, occupying the available adsorbent sites and “poisoning” the catalyst. A desulfation process can be part of a LNT regeneration process, and can be employed to remove the sulfur species and restore the NOx adsorption capacity of the LNT. The desulfation process typically occurs at a higher temperature than the NOx desorption process. For example, NOx desorption typically starts at a temperature of about 200° C. while desulfation typically starts at a temperature of at least about 500° C. Prior approaches to desulfating a LNT involve increasing the temperature of the engine exhaust stream to a sufficient temperature (by adjusting the operation of the engine) as well as typically introducing a fuel into the engine exhaust stream to provide further heating through catalytic combustion of the mixture promoted by a catalyst, preferably located upstream of the LNT.
Once the temperature is sufficiently high for LNT regeneration (for example, NOx desorption and/or desulfation) to occur, the engine exhaust stream is typically diverted away from the LNT in order to reduce the amount of oxygen present in the LNT, and create a reducing condition that facilitates regeneration.
In some of the regeneration processes described above, a liquid fuel, for example, diesel is introduced, and vaporized in the engine exhaust stream, then ignited over a DOC (or other catalyst within the after-treatment system) to provide heat for regeneration. However, during certain operating conditions of an engine, the temperature of the engine exhaust stream can be too low to adequately vaporize liquid diesel. For example, the vaporization of diesel generally requires a temperature greater than 250° C., yet the engine exhaust can be at a lower temperature. If the liquid diesel is not adequately vaporized or if vaporized diesel and exhaust mixture is not kept hot enough (resulting in condensation of diesel), the liquid fuel can potentially damage downstream after-treatment devices and/or systems, for example, causing hot spots, hydrocarbon carryover, or producing additional residues, carbon or particulates.
Instead of using diesel, a syngas stream comprising hydrogen (H2) and carbon monoxide (CO) can be employed as a fuel in the various regeneration processes described above. Employing syngas as a fuel for heating and/or to create a reducing condition during regeneration offers advantages. For example, because syngas ignites at a lower temperature than vaporized diesel, the threshold temperature required to initiate the regeneration processes can potentially be lowered. Also, typically regeneration can be performed using syngas without the need to alter the operating condition of the engine. Furthermore, with respect to LNT regeneration, higher NOx conversion efficiencies and desulfation efficiencies are typically achieved at lower temperatures in LNTs employing syngas relative to using diesel. If syngas is to be used, generally a fuel processor or syngas generator (SGG) is employed in the after-treatment system, and is sized to provide sufficient syngas output and/or heating duty for regeneration of one or more after-treatment devices in the system.
The present approach employs a syngas stream and then a combined fuel stream in a multi-stage process for regeneration of an engine exhaust after-treatment device. The combined fuel stream comprises a product stream from a syngas generator along with a supplemental fuel, such as diesel. Employing a combined fuel stream takes advantage of properties of both the product stream and the supplemental fuel, and can overcome at least some of the shortcomings of prior techniques.